In order to reduce the polluting emission, most internal combustion engines, principally Diesel engines, are equipped with an exhaust gas recirculation (EGR) system, for selectively routing back a part of the exhaust gas from the exhaust manifold of the internal combustion engine into the intake manifold of the internal combustion engine. The recirculated exhaust gas is mixed with fresh induction air and is fed into the engine cylinders, thereby reducing the production of oxides of nitrogen (NOx) during the combustion process.
Conventional EGR systems comprise an EGR conduit fluidly connecting the exhaust manifold to the intake manifold, an EGR cooler located in the EGR conduit for cooling the exhaust gas before mixing it with the induction air, and a valve for regulating the flow rate of the exhaust gas to be recirculated in the EGR conduit.
The temperature of the exhaust gas exiting from the EGR cooler is critical both to the NOx control process and to the integrity of the EGR cooler and the downstream components, such as EGR conduits, EGR flow control valves, and the engine.
In order to improve temperature control of the exhaust gas, some EGR systems may comprise two EGR coolers located in series along the EGR conduit. The first EGR cooler is designed to cool down the exhaust gas which comes at high temperature (up to about 700° C.) straight from the exhaust manifold, while the second EGR cooler receives the exhaust gas already partially cooled by the first EGR cooler and is designed to cool the exhaust gas at even lower temperature values (down to about 50° C.). In this way, the exhaust gas is subjected to two cooling stages, including a so called high-temperature cooling stage in the first EGR cooler and a so called low-temperature cooling stage in the second EGR cooler.
The first EGR cooler is usually connected to the cooling circuit of the internal combustion engine, so that the high-temperature cooling stage is performed with the aid of the same coolant used for cooling down other engine components, such as engine block and cylinder head. On the other hand, the second EGR cooler is connected to a dedicated coolant circuit, so that the low-temperature cooling stage is performed with the aid of a different coolant which can be maintained at a different temperature value. The dedicated coolant circuit generally comprises a main pump for circulating the coolant and a radiator for cooling the coolant once it has passed through the second EGR cooler.
In particular conditions, for example during the engine start-up phase in a cold environment, it could happen that the coolant used in the EGR cooler system is excessively cool. If such excessive cool coolant is supplied to the EGR cooler, the exhaust gas is also excessively cooled, resulting in an excessive decrease in temperature of the intake air. This may cause misfires in the internal combustion engine, adversely affecting the drivability. Furthermore, when the coolant temperature value drops below a predetermined temperature value, the vapor contained in the recirculated exhaust gas can condense in the second EGR cooler. This water condensation has a propensity to form aqueous acids when mixed with certain exhaust chemicals (such as, for example, a sulfur and nitrous oxide NOx). These acids can, over time, cause the corrosion of the components of the EGR system. In addition, when the temperature of the coolant is excessively low, the soot contained in exhaust gas is prone to settle on the interior surfaces of the EGR, causing the latter to be less efficient and leading to fouling and potential plugging.
It is therefore an object of the present invention to provide an EGR system that can accelerate the warm up of the coolant in the low-temperature cooling stage.
Another object is to provide an EGR system that can perform a finer and more stable temperature control of the low-temperature cooling stage.
Another object is to reduce NOx emission while keeping hydrocarbon (HC) and carbon oxides (CO) emission within acceptable levels, and protecting vehicle driveability.
Still another object is to achieve the above mentioned objects with a simple, rational and rather inexpensive solution.
These and other objects are achieved by a EGR system and an Internal Combustion Engine (ICE) system having the features recited in the independent claims. The dependent claims delineate preferred and/or especially advantageous aspects of the various embodiment of the invention.